The present invention relates in general to a liquid crystal display device, and, more particularly, the invention relates to a liquid crystal display device which includes a side-edge-type backlight that is capable of producing a high brightness by use of two or several linear light sources.
The liquid crystal display device is constituted of a pixel selection circuit and a liquid crystal panel in which a liquid crystal layer is interposed between two substrates, wherein a plurality of color filters are formed on one of the two substrates. With respect to the liquid crystal panel which constitutes the liquid crystal display device, various types of liquid crystal panels have been put into practical use. For example, an active matrix type liquid crystal display panel includes one substrate (also referred to as a TFT substrate), on which thin film transistors (TFT) for pixel selection are mounted, and another substrate (also referred to as a CF substrate) on which color filters are formed, the two substrates being laminated to each other. The TFT substrate includes a pixel region (a display region) which occupies a major portion of the TFT substrate and a drive circuit region and other circuit regions, which are arranged outside the pixel region. Here, although there is a known liquid crystal display device in which the color filters are arranged on the TFT substrate side, the following explanation is based on an example in which the color filters are formed on the above-mentioned CF substrate.
On the pixel region of the TFT substrate, usually, color pixels, each of which is constituted of sub pixels of three colors (red:R, green:G, blue:B), are arranged in a matrix array. Each sub pixel is constituted of a thin film transistor circuit (a pixel circuit). On the other hand, on the CF substrate, color filters of red:R, green:G, blue:B are formed, which color filters are usually arranged to face the respective sub pixels of three colors, which constitute the pixel region formed on the TFT substrate.
On the CF substrate, there is a light shielding film (a black matrix) which enhances the color purity and the contrast by preventing color mixture between the neighboring filters on a main surface of the transparent insulation substrate made of glass and the color filters of respective colors. Thereafter, a protective film (also referred to as a leveling film or an overcoat film) is formed on the surface of the transparent insulation substrate. Here, although an orientation film is directly formed on the protective film in a so-called IPS-type liquid crystal display device, transparent electrodes made of ITO or the like are formed on the protective film in a TN-type liquid crystal display device.
The liquid crystal display device which uses such a liquid crystal panel per se has no light emitting function. That is, on the basis of control performed on the drive circuit region, an electronic latent image is formed on the pixel region, and the latent image is visualized using light irradiated from an outside light source. Accordingly, the brightness (the luminance) of the display screen depends on the brightness (the luminance) of the outside light source.
With respect to the outside light source, a miniaturized liquid crystal display device, of the types which is mounted on an original mobile phone, a personal digital assistant or the like, makes use of external ambient light as an outside light source. However, a light source device (a backlight device), which is constituted of a light source, such as a light emitting diode or a cold cathode fluorescent tube (CFL), is usually mounted on a back surface of the liquid crystal panel. The backlight is roughly classified into a side-edge-type backlight and a direct type backlight. With respect to a liquid crystal display device which is used as a monitor, such as in a notebook type personal computer, to satisfy the demand for a reduction in thickness, the side-edge-type backlight has been widely adopted. The side-edge-type backlight is configured such that cold cathode fluorescent tubes, which constitute a linear light source, are arranged along one side, or two or more sides, of a light guide plate, which is made of a transparent material, such as acrylic resin, and light irradiated from the cold cathode fluorescent tubes is converted into a face light source by the light guide plate, so that a liquid crystal panel which is arranged above the light guide plate is illuminated with light produced from the face light source. In such a liquid crystal display device which uses a backlight, a general method of satisfying a demand for high luminance is to increase the number of the cold cathode fluorescent tubes which constitute the backlight. In many cases, one, two or more cold cathode fluorescent tubes are arranged on one side of the light guide plate.
FIG. 9 is a cross-sectional view showing the structure of part of a conventional liquid crystal display device in which two cold cathode fluorescent tubes are arranged on one side of a light guide plate. As seen in FIG. 9, in this liquid crystal display device, a backlight BL is mounted on a back surface of a liquid crystal panel PNL. Between the liquid crystal panel PNL and the backlight BL, there is an optical compensation sheet OPS, which is formed by stacking prism sheets and light diffusion sheets.
The liquid crystal panel PNL is formed by laminating two glass substrates SUB1, SUB2, and polarizers POL1, POL2 are laminated to respective surfaces of the glass substrates SUB1, SUB2. The backlight BL is constituted of a light guide plate GLB and two cold cathode fluorescent tubes CFL1, CFL2 which are arranged along the periphery on one side of the light guide plate GLB in a vertically overlapped manner. On a back surface of the light guide plate GLB, a reflection sheet RFS is mounted. A light reflection plate RFP is provided in such a manner that the light reflection plate RFP surrounds the three sides of the two cold cathode fluorescent tubes CFL1, CFL2 opposite to the light guide plate GLB.
The liquid crystal panel PNL, the backlight BL and the optical compensation sheet OPS are housed in a resin-molded intermediate casing MCS, and these parts are integrally fixed to each other and are formed into a module by an upper casing ACS, which covers the liquid crystal panel PNL side, and a lower casing BCS, which houses a back surface of the backlight BL. Here, symbols CBH1, CBH2 indicate power supply cables for the cold cathode fluorescent tubes CFL1, CFL2 and symbols GC1, GC2 indicate rubber cushions. In the explanation given hereinafter, the power supply cable is a part which is generally recognized as a cable and constitutes a wire which has a circular cross section, the outer periphery of the conductor of which is covered with an insulation material. Hereinafter, the power supply cable will be simply referred to as a cable.
FIG. 10A and FIG. 10B are views showing the rubber bushes which hold both ends of the cold cathode fluorescent tube and the pull-around arrangement of the power supply cable employed in this example, wherein FIG. 10A is a plan view in which the light guide plate is removed from the liquid crystal panel side in FIG. 9, and FIG. 10B is a side view as seen in the direction of an arrow G in FIG. 10A. Here, the cold cathode fluorescent tubes are not explicitly illustrated in the drawing, since illustration of the cold cathode fluorescent tubes makes the drawing complicated. Both ends of two cold cathode fluorescent tubes, which are accommodated in the inside of the light reflection plate RFP, are held in a given positional relationship by the rubber bushes GB1, GB2. On the end peripheries of the respective rubber bushes GB1, GB2, projections KB1, KB2 are formed which allow the rubber bushes GB1, GB2 to engage with the intermediate casing MCS or the like. Further, the respective cold cathode fluorescent tubes are provided with high-voltage-side cables CBH1, CBH2, which are connected with a high-voltage-side electrode terminal thereof, and low-voltage-side cables CBL1, CBL2, which are connected with low-voltage-side electrode terminal thereof.
FIG. 11A and FIG. 11B, which are similar to FIG. 10A and FIG. 10B, show how the high-voltage-side cable and the low-voltage-side cable of the cold cathode fluorescent tube are pulled around, in a state in which the rubber bushes shown in FIG. 10A and FIG. 10B are removed. FIG. 11a is a plan view, and FIG. 11B is a side view as seen in the direction of an arrow H in FIG. 11A. As seen in FIG. 11B, one end of the high-voltage-side cable CBH1 is connected to a high-voltage-side electrode terminal ELH1 of the cold cathode fluorescent tube CFL1, while one end of the low-voltage-side cable CBL1 is connected to a low-voltage-side electrode terminal ELL1 of the cold cathode fluorescent tube CFL1. Further, one end of the high-voltage-side cable CBH2 is connected to a high-voltage-side electrode terminal ELH2 of the cold cathode fluorescent tube CFL2, while one end of the low-voltage-side cable CBL2 is connected to a low-voltage-side electrode terminal ELL2 of the cold cathode fluorescent tube CFL2.
The high-voltage-side cables CBH1, CBH2 and the low-voltage-side cables CBL1, CBL2 are pulled around between a back surface of the lamp reflection plate RFP (a side opposite to the light guide plate with respect to the cold cathode fluorescent tubes) and an inner wall of the lower casing BCS in such a way that the high-voltage-side cables and the low-voltage-side cables have an equal length outside, and, thereafter, the high-voltage-side cables and the low-voltage-side cables are pulled out to the outside from the intermediate casing MCS.
Examples of the side-edge-type backlight which is provided with two or three cold cathode fluorescent tubes are described in the below-listed patent literatures 1 to 5. The patent literature 1 discloses a structure in which thermal interference of the electrode portion is avoided by longitudinally displacing two cold cathode fluorescent tubes having the same length. The patent literature 2 discloses a liquid crystal display device in which two or three cold cathode fluorescent tubes having the same diameter or different diameters are arranged in such a way that the cold cathode fluorescent tubes are stacked laterally, longitudinally, or longitudinally as well as laterally, on the side periphery of the light guide plate. Further, the patent literature 3 discloses a structure in which U-shaped cold cathode fluorescent tubes are arranged in an inclined manner, and the patent literature 4 discloses a structure in which there are three cold cathode fluorescent tubes which are respectively provided with lamp reflection plates. Further, the patent literature 5 discloses a structure in which damage to cables is presented by pulling out the power supply cables from non-cornered portions of a casing.
[Patent literature 1]
Japanese Patent Laid-Open 2003-234006
[Patent literature 2]
Japanese Patent Laid-Open Hei07(1995)-282613
[Patent literature 3]
Japanese Patent Laid-Open Hei06(1994)-109928
[Patent literature 4]
Japanese Patent Laid-Open 2001-75092
[Patent literature 5]
Japanese Patent Laid-Open 2001-222238